1. Technical Field
This invention generally relates to rotor blade assemblies for aircraft such as helicopters, and deals more particularly with a seal for protecting bearings in pitch joints of the blade assembly from contaminants.
2. Description of the Related Art
Helicopter aircraft employ a pitch joint that allows a main rotor blade to change its angle of attack or pitch. The ability to change the pitch on a rotor blade is essential to the operation of a helicopter. Pitch joints operate in a demanding mechanical environment due to high loads and rapid movement of the blades. Moreover, the loading on the pitch joint can reverse direction with every rotation of the rotor blade, depending upon whether the blade is advancing or retreating with respect to the movement of the helicopter.
The bearings in pitch joints often operate under hostile environmental conditions, and require lubrication in order to reduce wear. For maintenance and other reasons, dry lubricated bearing technologies are sometimes used to lubricate pitch bearings. Dry lubrication bearing technologies employ a low friction coating applied to a bearing metallic substrate that is one of two moving surfaces forming the bearing. The movements between the two surfaces wear away the coating on the bearing. During the wear, some of the low friction coating is transferred from the bearing to the mating part, which is often a sleeve. This transfer of the coating is essential for the correct operation of a dry lubricated bearing. In effect, the two moving surfaces of the bearing both become covered with low friction material during normal operation of the bearing system. Once the transfer of wear debris is established, the wear rate is dramatically reduced, and the life of the bearing is increased.
The reversing loading nature of the pitch joint places high demands on dry lubricated bearing technologies, especially when repeated gaps occur between the bearing surfaces as a result of loading reversal. Gaps between the bearing surfaces allow the wear debris to be flushed from the bearing joint, thus eliminating the transfer coating that has been transferred to the non-bearing member. Each time the pitch joint is flushed of wear debris, the system must redeposit a transfer coating to the non-bearing surface, thereby diminishing the amount of the low friction coating that remains on the bearing surface. As the low friction coating on the bearing wears, the gap that is created during load reversal increases over time, exacerbating the problem.
The problem of rapid wear of the low friction coating is further amplified where environmental contaminants such as water, sand or dust enter the joint and flush out the transfer debris at a faster rate than that caused by simple movement of air through the joint. In addition, debris such as sand and dust may become embedded in the relatively soft, low friction coating, thereby increasing friction which reduces the bearings' operating life.
Previous pitch joint designs have incorporated a seal directly on surfaces of the pitch joints. While this arrangement may be effective in some applications, it is difficult or impossible to add this type of seal construction after the pitch joint has been assembled, such as in a retrofit application.
Accordingly, there is a need for a bearing seal which effectively protects the bearing against environmental contaminants, yet which has mechanical properties and a physical configuration that does not interfere with the normal movement and operation of the pitch joint. The present invention is directed toward satisfying this need.